1. Field of the Invention
This invention relates to a system and method for encoding information onto an optical beam, and more particularly to the encoding of information onto an optical beam transmitted through a photorefractive material in devices such as phase conjugate mirrors.
2. Description of the Prior Art
Photorefractive materials have been used in a number of different applications involving the processing of optical beams. One principal application is in phase conjugate mirrors (PCMs). Other applications include holography, image processing and the performance of optical mathematical functions such as image amplification, pattern subtraction and pattern recognition. In each of these applications the photorefractive optical system has been essentially passive in the sense that it has a known response to an input optical beam, and in effect produces an output beam that is slaved to the input beam.
Since PCMs and photorefractive devices in general are of interest in this invention, it will be helpful to briefly review some of their basic operating and structural characteristics.
In general, a photorefractive material is one in which the index of refraction changes under the influence of applied light, such as a laser beam. The light causes charges within the photorefractive material to migrate and separate, producing an internal electrostatic field. This field produces a change in the crystal's refractive index by the linear electro-optic effect (the Pockels effect). The theory of the electro-optic effect is described in a text by A. Yariv, "Introduction to Optical Electronics, 2d ed.", pp. 246-53 (1976). The photorefractive "index grating", or periodic variation in the crystal's index of refraction, is a measure of the change in the index. Photorefractive materials generally comprise III-V and II-VI semiconductor combinations within the periodic table, and other crystals such as BaTiO.sub.3, Bi.sub.12 SiO.sub.20 and KTa.sub.1-x Nb.sub.x O.sub.3.
The formation of a photorefractive index grating is illustrated in FIG. 1, in which the horizontal axis represents distance within the photorefractive crystal. The upper graph illustrates the pattern of light with a spatially periodic intensity I within the crystal, while the next graph illustrates the resulting charge density .rho. within the crystal. The mobile charges, illustrated as being of positive polarity, tend to accumulate in the dark regions of the light intensity pattern. The resulting periodic charge distribution produces a periodic electrostatic field E by Poisson's equation. This electric field, illustrated in the third graph of FIG. 1, then causes a change in the refractive index .DELTA.n of the crystal by the linear electro-optic effect. The electro-optic coefficient is proportional to the ratio of the refractive index change to the space charge electrostatic field within the crystal material. The photorefractive effect, illustrated in the last graph of FIG. 1, is non-local in that the maximum refractive index change does not occur at the peak of the light intensity. In FIG. 1 the spatial shift between .DELTA.n and I is 1/4 (a 90.degree. phase shift) of the grating period; in general, however, this shift can be any fraction of the grating period.
Phase conjugation is an optical phenomenon that has attracted considerable attention in recent years. Several different ways of producing phase conjugated beams have been discussed in the literature, including four-wave mixing, stimulated Brillouin scattering, Raman scattering, three-wave mixing and photon echo devices. A review of various applications of optical phase conjugation is presented by Giuliano in Physics Today, "Applications of Optical Phase Conjugation", April 1981, pages 27-35. A general review of the field is given in A. Yariv, IEEE, J. Quantum Electronics QE14, 650 (1978), and in "The Laser Handbook Vol. 4", edited by M. L. Stitch and M. Boss, Chapter 4 by the present inventor, "Non-Linear Optical Phase Conjugation", pages 333-485, North Holland Publishing Co. 1985.
Basically, a phase conjugate mirror (PCM) produces a retro-reflective reflection of an incident beam, with the phase of the reflected beam reversed from that of the incident beam at the point of reflection. A typical PCM known in the prior art is shown in FIG. 2. This is illustrated as a four-wave mixer, in which a pair of contradirectional laser beams 2 and 4 are directed into an optical mixing medium 6. An initializing laser beam E.sub.I, equal in frequency to beams 2 and 4, is directed into the mixing medium from the side. As a result of the action of the various beams within the mixing medium, a reflected beam RE.sub.I *, where R is the coefficient of reflectivity, is reflected back in a direction opposite to incident beam E.sub.I. Since power is pumped into the system by beams 2 and 4, the reflector may produce an amplification which makes R greater than 1.
In addition to being retro-reflective to the incident beam, the phase conjugated reflected beam also undergoes a phase reversal with respect to the incident beam at the point of reflection. This is illustrated in the phase diagram of FIG. 3, which depicts the incident and reflected waves as vectors plotted against a horizontal real axis and a vertical imaginary axis. It may be seen that the phase angle of the reflected beam RE.sub.I * is equal in absolute magnitude but reversed in polarity from the incident beam E.sub.I.
PCMs can be provided either with external pumping beams, as in the four-wave mixer illustrated in FIG. 1, or as a "self-pumped" device which eliminates the requirement for external pump beams. In one application external information has been encoded onto the output beam of a four-wave mixer by modulating the pump beams, e.g., H. I. Mandelberg, "Phase Modulated Conjugate Wave Generation in Ruby", Optics Letters, Vol. 5, p. 258, 1980. However, this technique requires the use of both external pump beams and an external modulator.
Of the self-pumped PCMs, those employing Brillouin or Raman scattering are generally employed in connection with high power pulsed laser beams, such as from a Nd:YAG laser, but are not practical with low power continuously operating lasers such as HeNe devices. Another type of self-pumped PCM is based upon the use of a photorefractive material, which usually possesses a high electro-optic coefficient as the phase conjugating medium. Such a self-pumped PCM has been employed with continuously operating, low-power lasers such as HeNe lasers.
While photorefractive materials have been used as PCMs and other devices to produce a known response to an input optical beam, they have not been used to transmit additional information. Since many of these devices constitute effective communications channels, they are not being fully utilized when their use is thus restricted to the limited purposes for which they were originally developed.